EFFECT OF ROCK PERMEABILITY ON THE FORMATION OF DIAGNOSTIC ARGILLIC AND FRAGIPAN HORIZONS

Leached soils (Alfisols) with either an argillic horizon or a fragipan were encountered close to each other on the marly-limestone and sandstone plateaus of Haute-Saone (France). Soils have formed under periglacial conditions during the Riss in a similar climatic environment. Because the soils occurring today are in the same geomorphic and topographic position, the objectives of the investigations were to study the influence of other pedogenic factors, such as parent materials and underlying rock type. The soil mantle, above the bedrock, is from one and a half to several meters thick. Four types of materials were identified through the pedons studied: sands, clays, clay-loams, loams. Their location being directly above either a rhetian sandstone or a sinemurian marly-limestone, sandy or clayey materials were accumulated as insoluble residue of the underlying rocks. Related to particle size distribution and mineralogy, the clay loamy materials were identified as eolian dusts. The loamy materials were also eolian deposits, but they were reworked and transported with sandy particles from sandstone of other high landscape positions. The surficial materials, encountered in all the sites studied, were texturally and mineralogically similar, which suggested that the parent materials were not the main factor controlling the formation of argillic horizons or fragipans. On the other hand, the effect of rock type has been established. Our data suggest: (1) significant relationships between the internal drainage of soil and the underlying geological substrata (type, joints and fracture planes, permeability, effect on hydrology), (2) rock effect on formation of an argillic or a fragipan horizon. In the case where meteoric waters can move through fractured rocks, soils can undergo drying and wetting cycles and clays swell and shrink. These processes induce favorable effects on genesis of soil structure, particularly of Bt structure. In the case of low-porosity and low-permeability rocks, soils dry from the surface downward and dessication cycles accomplish a vertical cleavage and polygonal cracking. We have concluded that the rock effect is a major factor involved in the development of leached soils with an argillic horizon or a fragipan. A major advantage of the rock factor hypothesis is that the formation of fragipans is possible in periglacial areas and far from glaciated regions, as proposed by the literature.